The properties of complex technical systems are nearly always affected by the temperature of its elements or components. For example, the temperature influences the performance, the service life or also the safety state of such systems. Complex technical systems therefore often require checks and active adjustments of their temperature. A temperature measurement and active temperature adjustment is of particular importance for systems whose function is based on electrochemical, chemical and/or physical processes. One example of a temperature-sensitive, complex technical system is a storage device for electrical energy, such as galvanic cells, e.g., lithium-ion cells, NiMH cells or lithium-polymer cells, and batteries. Batteries are galvanic cells connected in series and/or parallel, but accumulators may be considered batteries as well within the meaning of the present invention. Accumulators are multiple-use storage devices for electrical energy, often based on an electrochemical system, and therefore constitute a special type of battery. In contrast to non-rechargeable batteries made up of primary cells, an accumulator is made up of one or more rechargeable secondary secondary cell(s). As in the case of batteries, a plurality of cells may be connected in series in order to increase the overall voltage, or they may be connected in parallel to increase the capacitance. In addition to electrochemical cells, capacitors may also be used as storage cells for electrical energy, as well as hybrids made up of capacitors and electrochemical cells.
Storage cells for electrical energy, especially batteries, for example, are used to supply electrical energy to portable electrical devices, e.g., mobile telephones, laptops, camcorders, MP3 players, electric vehicles (EVs), PHEV (plug-in hybrid electrical vehicles), HEVs (hybrid electrical vehicles) or E-bikes, and stationary systems such as photovoltaic systems or network buffers.
The performance and service life of storage cells for electrical energy frequently depend heavily on their electrical, thermal and mechanical loading. For example, the individual cells of a battery degrade if the battery is operated within an unfavorable temperature range. In order to minimize a degradation of its individual cells in the course of the operational life of the battery, the actual temperature of the battery or its cells, that is, the temperature that the battery or its cells has or have at any given time, should not exceed a preferred operating temperature of the battery, i.e., the setpoint temperature. Preferred operating temperatures for NiMH batteries are between 10 to 55° C., for Li-ion batteries, between 10 to 40° C. Not only when operating a battery from which the stored energy is withdrawn, but also during charge processes of the multi-use batteries, the actual temperature may exceed the setpoint temperature if no separate cooling of the battery or its individual cells takes place.
Batteries are often cooled in order to ensure that the actual temperature of a battery does not rise beyond a setpoint temperature, so that at least their thermal loading is reduced during operation.
German Published Patent Application No. 10 2007 017 172 describes a cooling system for a unit to be cooled, which has a coolant circuit having a coolant able to be recirculated, which is set up in such a way that the recirculation direction of the coolant is reversible after at least one predefined time interval or according to a control that is based on a temperature of the cooling-requiring unit. The cooling-requiring unit may be a battery, power electronics, an electric motor or a fuel cell.
However, achieving the optimal output of a battery requires more than ensuring that the battery does not overheat and cooling it, so that its actual temperature does not exceed its setpoint temperature. To allow the most optimal exploitation of the battery capacity, especially at the beginning of its operation when the actual temperature of the battery is lower than its setpoint temperature, it may be useful to warm the battery in order to thereby reduce its internal resistance. In addition, safety-critical states may arise in lithium-ion cells if the cells are charged at temperatures that are too low in relation to the respective current. In addition to charging the lithium-ion cells at a charge station, this is important especially also for the recuperation, such as while driving.
Only when the battery has warmed up in the course of its operation and its actual temperature approaches the setpoint temperature will no further heating of the battery be required any longer.